Artificer's armorer subclass features and magic armor : Armor modifications & Infusions

Question

Many of the features of the Artificer's Armorer subclass published in Tasha's Cauldron directly impact their armor, and I have had a few questions on how these features would work in relation to magic armors.

The stack has already established that the level 3 feature Arcane Armor does indeed work on magic armors, so I am moving to the next one : Armor modifications

That feature describes that:

At 9th level, you learn how to use your artificer infusions to specially modify your Arcane Armor. That armor now counts as separate items for the purposes of your Infuse Items feature: armor (the chest piece), boots, helmet, and the armor's special weapon

My question on that feature has to do with the Artificer's Infusion feature, that specifies that it works only on mundane items.
Because of that, my interpretation was that while you could use a magic armor for your arcane armor feature, it would mean that you would be unable to put infusions on it even when reaching 9th level, since the "magic property" of your armor would be carried over to each of the armor parts.
However while searching the web, I found many people arguing otherwise, and am now unsure about my reasoning.

Does the armor modification feature make you able to infuse the "extra" parts (e.g. boots or weapon) of your magic Arcane armor, or does the "magic" property of the armor carry over on those parts and make you unable to do so?

Answer 1

First, I agree with your interpretation: You can't infuse magic armor, whether it's your Arcane Armor or not.

However, since it says the chestpiece counts as the "armor" part for the purpose of infusion, I would rule that you can't infuse the chestpiece, but you can infuse the gauntlets, boots, etc.

Naturally, if you ever donned a new suit of armor (which ends the Arcane Armor effect) the old suit would immediately lose all its infusions, since the gauntlets, boots, etc, no longer count as separate items. But that would happen whether it's magic armor or not, except that non-magical armor could theoretically retain the chestplate infusion.